1. Field of the Invention
The present invention relates to a power amplifier. More particularly, the present invention relates to a power amplifier comprising a plurality of power amplifier units operating in different class regions.
2. Description of Related Art
Wireless communication is without a doubt a popular research and commercial topic. Typically, wireless communication starts from a transmitter processing input signals to output signals that then transmit “wirelessly” to a receiver. Power amplifier (PA) is an important device in a radio frequency (RF) front-end circuit for a transmitter. Specifically, a power amplifier is used to amplify the broadcast signals at the transmission (TX) terminal of a radio frequency circuit. With the popularity of portable communication systems, maximum output power and output power efficiency has become the critical factors for the development of wireless transmission.
A conventional linear power amplifier, such as a class-A, a class-B or a class-AB power amplifier, biases an active device (the power amplifier) thereof by a fixed DC current , so as to obtain better linearity even though efficiency of the conventional power amplifier is poor due to a full-time DC power consumption. Moreover, when an input power is excessively significant, an output power thereof cannot be linearly amplified, which causes a gain compression and a signal distortion. At this time, a signal average output power has to be reduced to maintain the linearity, which is referred to as power back-off, so that the power amplifier cannot be operated in a high-efficiency interval, and an average efficiency of the power amplifier is reduced.
More specifically, in order to obtain better linearity performance, the DC current should be increased to enable the power amplifier operating in a class-A operating region. In the other hand, in order to increase output power efficiency of a power amplifier, the power amplifier sometimes operates in a class-AB or class-B operating region. In other words, the gate bias voltage of the power amplifier may be lowered. Namely, the current consumption and the linearity performance of the power amplifier are trade off with each other. Hence, how to enhance efficiency performance and lower current consumption of a power amplifier without reducing the maximum output power and linearity of operation of the device is important and required.